技术分享 | SpringBoot 流式输出时，正常输出后为何突然报错？

项目背景

1. 一个 SpringBoot 项目同时使用了 Tomcat 的过滤器和 Spring 的拦截器，一些线程变量在过滤器中初始化并在拦截器中使用。
2. 该项目需要调用大语言模型进行流式输出。
3. 项目中，笔者使用 SpringBoot 的 ResponseEntity<StreamingResponseBody> 将流式输出返回前端。

问题出现

问题出现在上述第 3 点：正常输出一段内容后，后台突然报错，而报错内容由拦截器产生。

笔者仔细查看了报错日志，发现只是拦截器的问题：执行时由于某些线程变量不存在而报错。但是，这些线程变量已经在过滤器中初始化了。

那么问题来了：为什么这个接口明明可以正常通过过滤器和拦截器，并开始正常输出，却又突然在拦截器中报错呢？

场景重现

Filter

@Slf4j @Component @Order(1) public class MyFilter implements Filter { @Override public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException { // 要继续处理请求，必须添加 filterChain.doFilter() log.info("doFilter method is running..., thread: {}, dispatcherType: {}", Thread.currentThread(), servletRequest.getDispatcherType()); filterChain.doFilter(servletRequest,servletResponse); } } 

Interceptor

@Slf4j public class MyInterceptor implements HandlerInterceptor { @Override public boolean preHandle(@NotNull HttpServletRequest request, @NotNull HttpServletResponse response, @NotNull Object handler) throws Exception { log.info("preHandle method is running..., thread: {}, dispatcherType: {}", Thread.currentThread(), request.getDispatcherType()); if (DispatcherType.ASYNC == request.getDispatcherType()) { log.info("preHandle dispatcherType={}", request.getDispatcherType()); } return true; } @Override public void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView) throws Exception { log.info("postHandle method is running..., thread: {}", Thread.currentThread()); } @Override public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception { log.info("afterCompletion method is running..., thread: {}", Thread.currentThread()); } } 

WebMvcConfigurer

@Configuration public class WebAppConfigurer implements WebMvcConfigurer { @Bean public MyInterceptor myInterceptor() { return new MyInterceptor(); } @Override public void addInterceptors(InterceptorRegistry registry) { registry.addInterceptor(myInterceptor()).addPathPatterns("/**"); } @Override public void configureAsyncSupport(AsyncSupportConfigurer configurer) { configurer.setDefaultTimeout(120_000L); configurer.registerCallableInterceptors(); configurer.registerDeferredResultInterceptors(); ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor(); executor.setCorePoolSize(5); executor.setMaxPoolSize(10); executor.setQueueCapacity(100); executor.setThreadNamePrefix("web-async-"); executor.initialize(); configurer.setTaskExecutor(executor); } } 

Controller

@Slf4j @RestController @RequestMapping("/test-stream") public class TestStreamController { @ApiOperation("流式输出示例") @PostMapping(value = "/example", produces = MediaType.TEXT_EVENT_STREAM_VALUE) public ResponseEntity<StreamingResponseBody> example() { log.info("Stream method is running, thread: {}", Thread.currentThread()); return ResponseEntity.status(HttpStatus.OK) .contentType(new MediaType(MediaType.TEXT_EVENT_STREAM, StandardCharsets.UTF_8)) .body(outputStream -> { log.info("Internal stream method is running, thread: {}", Thread.currentThread()); try (outputStream) { String msg = "To be or not to be!"; outputStream.write(msg.getBytes(StandardCharsets.UTF_8)); outputStream.flush(); } }); } } 

根据以下运行日志，我们可以看到拦截器的 preHandle 确实执行了两次，并且此次调用过程共有 3 个线程（io-14000-exec-1，web-async-1，io-14000-exec-2）参与了工作。

2024-05-06 07:35:27.362 INFO 209108 --- [io-14000-exec-1] o.a.c.c.C.[.[localhost].[/java-study] : Initializing Spring DispatcherServlet 'dispatcherServlet' 2024-05-06 07:35:27.362 INFO 209108 --- [io-14000-exec-1] o.s.web.servlet.DispatcherServlet : Initializing Servlet 'dispatcherServlet' 2024-05-06 07:35:27.365 INFO 209108 --- [io-14000-exec-1] o.s.web.servlet.DispatcherServlet : Completed initialization in 3 ms 2024-05-06 07:35:27.402 INFO 209108 --- [io-14000-exec-1] com.peng.java.study.web.config.MyFilter : doFilter method is running..., thread: Thread[http-nio-14000-exec-1,5,main], dispatcherType: REQUEST 2024-05-06 07:35:28.107 INFO 209108 --- [io-14000-exec-1] c.p.java.study.web.config.MyInterceptor : preHandle method is running..., thread: Thread[http-nio-14000-exec-1,5,main], dispatcherType: REQUEST 2024-05-06 07:35:28.121 INFO 209108 --- [io-14000-exec-1] c.p.j.s.w.r.test.TestStreamController : Stream method is running, thread: Thread[http-nio-14000-exec-1,5,main] 2024-05-06 07:35:28.152 INFO 209108 --- [ web-async-1] c.p.j.s.w.r.test.TestStreamController : Internal stream method is running, thread: Thread[web-async-1,5,main] 2024-05-06 07:35:28.167 INFO 209108 --- [io-14000-exec-2] c.p.java.study.web.config.MyInterceptor : preHandle method is running..., thread: Thread[http-nio-14000-exec-2,5,main], dispatcherType: ASYNC 2024-05-06 07:35:28.167 INFO 209108 --- [io-14000-exec-2] c.p.java.study.web.config.MyInterceptor : preHandle dispatcherType=ASYNC 2024-05-06 07:35:28.174 INFO 209108 --- [io-14000-exec-2] c.p.java.study.web.config.MyInterceptor : postHandle method is running..., thread: Thread[http-nio-14000-exec-2,5,main] 2024-05-06 07:35:28.183 INFO 209108 --- [io-14000-exec-2] c.p.java.study.web.config.MyInterceptor : afterCompletion method is running..., thread: Thread[http-nio-14000-exec-2,5,main] 

问题分析

1. 方法调用流程的差异

众所周知，SpringBoot 的普通输出接口调用流程图如图 1 所示。

（图1-SpringBoot 普通输出调用流程图）

结合日志，我们可以简单画出流式输出接口对应的流程图（图 2）。

（图2-SpringBoot 流式输出调用流程图）

2. 线程的差异

普通接口的执行时序图如图 3 所示。

（图3-普通接口的时序图）

而流式接口的时序图如图 4 所示。

（图4-流式接口的调用时序图）

解决问题

通过分析，对流式输出的情况提出两种解决方案：

1. 将过滤器中的部分业务逻辑迁移到拦截器中。
2. 根据条件，跳过第二次的拦截器 preHandle 方法。

笔者选择了第二个方案，实现代码如下。

@Slf4j public class MyInterceptor implements HandlerInterceptor { @Override public boolean preHandle(@NotNull HttpServletRequest request, @NotNull HttpServletResponse response, @NotNull Object handler) throws Exception { log.info("preHandle method is running..., thread: {}, dispatcherType: {}", Thread.currentThread(), request.getDispatcherType()); // 如果是异步请求，则跳过 if (DispatcherType.ASYNC == request.getDispatcherType()) { log.info("preHandle dispatcherType={}", request.getDispatcherType()); return true; } return true; } @Override public void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView) throws Exception { log.info("postHandle method is running..., thread: {}", Thread.currentThread()); } @Override public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception { log.info("afterCompletion method is running..., thread: {}", Thread.currentThread()); } } 

需要注意，请求线程和回调线程都需考虑清理线程变量，不然会导致内存泄漏。

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